This invention generally relates to the production of high performance castings. In particular, the invention is directed to techniques for producing turbine components and other superalloy, ferrous, or titanium articles which are subjected to similar operating conditions.
Turbine components, for example turbine wheels, are subjected to operating conditions which place great demand upon the components. Thus, it is well known that the temperature and atmospheric conditions to which turbine components are subjected require properties in the components which will insure suitably consistent performance for a reasonably long period of time.
Turbine wheels lead to particular production problems since the blade sections of such wheels are subjected to stresses and other operating conditions which are distinct from the conditions to which the disc section of the wheels are exposed. Cast turbine wheels have been produced in an integral fashion; however, such wheels cannot be produced consistently with desired properties. In particular, the disc sections of the integral castings do not achieve desired low-cycle fatigue behavior even though the cast blade sections might be suitable.
Composite turbine wheels are produced involving the separate formation of blades through the use of precision casting operations. The disc sections of the wheels are separately formed, forging operations being utilized for this purpose. The blades are then connected to the disc section, usually by mechanical means, and the composite structure provides a suitable combination. Thus, the cast structure of the blades is suitable for the conditions to which the blades are exposed while the forged structure of the discs provides suitable properties in this area.
The production of composite turbine components leads to other problems, however, for example the additional steps involved and the necessity for insuring that precision machining operations and the like are properly conducted. Composite structures thus lead to additional expense when compared with structures which can be produced integrally. Also, many designs are limited by the rim space available for blade attachment precluding the use of composite turbine components.
Hot isostatic pressing has also been proposed as a means for improving the properties of superalloy turbine components, for example as described in Freeman, et. al. U.S. Pat. No. 4,021,910 issued on May 10, 1977. The refinement of grain size for improving fatigue capability has also been proposed including the use of nucleants in the facecoat of ceramic molds for producing fine grained castings. However, this process is not capable of refining grains to the extent exhibited by forgings and achieving significant refinement in heavy sections is particularly difficult.